1. Field of the Invention
The present invention is generally related to fuel elements for nuclear reactor cores and, in particular, to a holddown spring arrangement for securing fuel assemblies in the nuclear reactor core.
2. General Background
Typically, a nuclear reactor for the generation of electrical power includes a core of fissionable material to heat a coolant flowing up therethrough. The fissionable material is enclosed in elongated fuel rods assembled in a square array called fuel assemblies. The fuel rods are held in spaced parallel relationship by a number of spacer grids distributed at intervals along the length of the assembly. The fuel assemblies are held in an array by core grid plates at the top and bottom and are provided with upper and lower end fittings for mating with the grid plates. Typically, hold down spring means is provided between the upper end fitting and the upper core grid plate. This is necessary to provide sufficient hold down force against hydraulic lift forces in the core generated by coolant flow. The springs also allow for axial dimensional growth of the fuel assembly due to either differential thermal expansion or irradiation induced material change. The problems of design, therefore, are in the ability to provide sufficient hold down force against hydraulic lift while allowing sufficient room for growth. Sufficient material strength and stiffness must be available within a limited volume area. The stiffness/volume efficiency of a spring becomes very important when used for nuclear fuel hold down.
Known devices which address this problem include the following.
U.S. Pat. No. 4,551,300 entitled "Nuclear Reactor Fuel Assembly End Fitting" discloses an end fitting having a plurality of rigid levers and elastic means in recesses in the end fitting which exert a restoring torque on corresponding levels.
U.S. Pat. Nos. 4,072,562 and 4,072,564 disclose the use of torsion bars as hold down means.
U.S. Pat. Nos. 4,671,924; 3,801,453; 4,427,624; 4,631,166; 4,420,457; and 4,560,532 disclose the use of leaf springs as hold down means.
U.S. Pat. Re. Nos. 31,583; 3,475,273; 3,515,638; 3,600,276; 3,689,358; 3,770,583; 4,076,586; 4,078,969; 4,192,716; 4,208,249; 4,278,501; 4,534,933; 4,551,300; and 4,729,868 disclose the use of a variety of hold down devices including helical springs and are representative of the general state of the art.
The known devices leave room for improvement. Leaf springs tend to be stroke limited and often must be ganged to achieve sufficient hold down force. As fuel assemblies are typically only eight inches square, there is generally inadequate dimension available to obtain sufficient flexure of the bar. Helical springs must be positioned to allow access of control rods and are fully exposed to coolant flow which subjects the springs to the dynamic stresses of flow induced vibration. Also, reconstitution of a fuel assembly utilizing the helical spring design is a relatively complex operation.